The present invention relates generally to an apparatus and method for converting carbonaceous particles into a fuel gas. More particularly, the present invention relates to an improved coal conversion apparatus and method whereby prior pretreatment of the coal in a separate processing zone is substantially avoided.
In the gasification of coal, it is well known that certain coals, particularly bituminous coal, become plastic and sticky under certain temperature conditions. In this plastic state, the coal particles can cake or agglomerate. This caking or agglomeration interferes with gasification and consequently must be substantially reduced or eliminated for an efficient gasification process.
Various methods to reduce agglomeration are known. As disclosed in U.S. Pat. No. 2,805,189, the coal particles can be mildly oxidized with a gas containing oxygen, such as air. This method has the disadvantages of generating large amounts of heat that are difficult to recover and converting a portion of the carbon value of the coal to carbon dioxide. In addition, the gaseous byproducts of the pretreatment step contain valuable hydrocarbon liquids which are lost, unless separately treated, from the product gases. This, however, substantially increases the capital expenditure for the gasifier unit.
Another method of pretreating coal is illustrated in U.S. Pat. No. 2,582,712. A single volume of raw pulverized feed coal is admixed with 15 to 30 volumes of hot circulating residue recovered from the gasification reaction. This rapidly preheats the coal to gasification temperatures and, therefore, substantially avoids coal agglomeration. However, the circulation of high volumes of abrasive solid particulate matter has the obvious disadvantages of equipment wear and expense.
A method of introducing coal directly into a fluidized bed is illustrated in U.S. Pat. No. 3,927,996. In this method, coal is entrained in a carrier gas and, injected at a superficial gas velocity of 15-1,000 ft. per second at an acute angle down into the upper or cooler portion of the fluid bed. The velocity of the carrier gas is adjusted in response to the amount of fines carried overhead to control fines loss. This adjustment of carrier gas velocity and the preferred introduction of the coal into the upper portion of the bed at a high velocity are not conducive to the efficient conversion of coal to a fuel gas and the maintenance of a stable fluidized bed in the reactor.
Finally, there is illustrated in U.S. Pat. No. 2,577,632 a method for introducing coal particles into a fluidized bed from a single injection point directly into the center of a fluidized bed at a slow velocity. This method of introduction does not include high velocity injection and, as a result, increases the tendency for the coal to agglomerate in the fluidized bed.